In the past, several drugs having a tricyclic structure have been found useful in the treatment of severe psychotic disorders, especially of the schizophrenic type. Some of these drugs are thioxanthenes which are substituted in the 2-position of one of the benzene rings, and some of the most active are described in U.S. Pat. No. 3,116,291. Recently some thioxanthenes having a fluoro atom in the 6-position have been described, for example in U.S. Pat. No. 4,042,695 as having neuroleptic properties of the same level as the known thioxanthene-neuroleptics but a much lower level of pharmacological effects associated with extrapyrimidal symptoms. Further, some piperidylidene-thioxanthene derivatives having almost no extrapyrimidal side effects have been described in Belgian Pat. No. 835,224. However, the said known thioxanthene compounds which have the lowest extrapyrimidal side effects also have relatively short-acting neuroleptic effects.